What is described are magneto-resistive (MR) head pre-amplifier designs for use with hard disk drives (HDDs). In particular, differential MR head pre-amplifiers for use with single polarity power supply applications are presented.
MR heads operate based on a physical phenomenon known as the magneto-resistive effect. Certain metals, when exposed to a magnetic field, change their resistance to the flow of electricity. This property has been exploited in creating heads for reading data stored in the HDDs.
Conventional HDDs include magnetic rotating disks (or platters) that are typically made of glass or ceramic, and have a layer of magnetic material deposited on their surface. Data is stored on the platters in the form of binary digits sent to the HDD in a time sequence of binary “one” and “zero” values, or bits. Areas of the rotating platters are magnetized in opposite directions depending on the data to be stored. A typical data storage scheme results in “ones” being represented by reversals in magnetic polarization stored on the surface of the platters, with “zeroes” being stored in areas of the platters between the areas where the “one” values are stored.
In conventional HDDs, an MR head is arranged in close proximity to the rotating platters to read the magnetically stored data. Typically, a bias current generator supplies a constant bias current to the MR head. When the MR head passes over a magnetic field on one of the rotating platters, the head changes its resistance. As the bias current is fixed, variations in the resistance of the MR head lead to corresponding changes in the voltage produced across the MR head. Typically, a read pre-amplifier is coupled to the MR head to amplify the relatively small voltage produced across the MR head to an appropriate level for use by other components in the HDD assembly. Pre-amplifiers having a gain of more than a hundred are typical.
MR head pre-amplifier designs may be grouped broadly into two main categories: differential and single-ended pre-amplifier designs. In differential designs, each terminal of the MR head is fed to respective inputs of the amplifier. In contrast, single-ended designs have only one terminal of the MR head being fed to the single amplifier input (see, e.g., U.S. Pat. No. 6,219,195).
Differential MR head pre-amplifiers typically exhibit superior common-mode noise rejection (CMNR) characteristics over their counterpart single-ended designs. Noise signals typically occur equally at both terminals of the MR head, and tend to cancel one another out in differential designs. With single-ended designs, the unwanted noise signals are amplified by the read pre-amplifier, possibly resulting in errors in the recovered data. CMNR is thus an important design characteristic in establishing the maximum performance capability of the HDD.
A drawback of differential pre-amplifier designs is that the amplifiers typically require both a positive and negative power supply in order to maintain a differential MR head voltage potential that is near zero volts. Typically, the nominal differential head voltage is chosen to be about ±100 mV. Maintaining a near-zero head potential is important to protecting the sensitive MR head from electrical damage should the head come into contact with the platters during operation. But having both a positive and negative power supply adds significant cost and design complexity to the overall HDD design.
The added cost and complexity mainly result from the need to provide the HDD with an additional voltage regulator for generating the negative supply voltage. Typically, all other components in the HDD are capable of being biased using only a positive power supply voltage. Thus, there is a need for a differential pre-amplifier design that maintains the MR head voltage near zero volts without requiring a negative power supply voltage.
Another design constraint associated with MR head pre-amplifier design results from passing a constant bias current through the MR head. This bias current consequently produces a relatively large DC bias voltage (e.g., hundreds of millivolts) across the MR head that tends to saturate the input(s) of the pre-amplifier. Conventionally, coupling capacitors are arranged between the MR head terminal(s) and the amplifier input(s) to remove the DC component of the MR head voltage (see, e.g., U.S. Pat. Nos. 6,219,195 and 6,252,735).
But the relatively large magnitude of these coupling capacitors, and their associated parasitic capacitances, can produce undesired gain peaking in the frequency response of the amplifier. Gain peaking tends to increase the bandwidth of the closed-loop system at the expense of decreased circuit stability. The coupling capacitors can also introduce so-called corner frequencies into the frequency response of the amplifier, causing the gain to roll off below these frequencies. U.S. Pat. No. 6,084,460 describes a differential MR pre-amplifier having an arrangement that compensates for corner frequency roll off, but fails to address the problem of gain peaking. Thus, there is also a need for a single-supply differential pre-amplifier design that is AC-coupled to the MR head, but having a lowered input capacitance to reduce the effect of gain peaking in the frequency response of the amplifier.